Electrical apparatus

ABSTRACT

A semiconductor switch die (30) is mounted on one face of a substrate (31) and has a temperature sensor (10) on the opposite face to monitor the temperature in the vicinity of the semiconductor die. A current sensor (4) and voltage sensor (5) supply signals to a processor (15) to calculate power dissipated by the semiconductor device. The processor calculates the temperature of the device itself from the measured temperature and the dissipated power. If this exceeds a preset limit, the switching device is opened to prevent damage.

This invention relates to electrical apparatus of the kind including asemiconductor switching device, an arrangement that derives a measure ofpower dissipated by said device, a sensor that provides a measure of thetemperature in the vicinity of the device, and means to restrict flow ofcurrent through the device in response to a combination of the measureof power and the measure of temperature.

The usual way of protecting electrical equipment and its wiring fromcurrent overloads is by means of an electromagnetic relay switch,thermal wire fuse or electrothermal circuit breaker. These previousarrangements, however, are usually relatively heavy and bulky. They alsohave a slow response time and can be unreliable in the long term.Because of this, there is a move towards using solid state powercontrollers (SSPC) employing power semiconductors, to switch electricalenergy to a load and to interrupt current flow should an overload bedetected.

The advantages of solid state controllers over electromechanicalcounterparts are well recognised and typically include factors such asreduced weight, reduced time to respond, increased lifetime andincreased reliability.

Despite these advantages, using power semiconductors in such a role doesrequire a great deal of care because of the adverse effect of hightemperatures on semiconductor devices. The maximum temperature fordevices made of silicon is about 175° C. and for semiconductors made ofsilicon carbide it is about 400° C. In current limiting SSPC's, thepower density within each die may exceed 500 W for short periods,resulting in rapid heating of the die and surrounding area.

In U.S. Pat. No. 4,937,697 there is described a circuit for protectingan FET device. The circuit employs a reference circuit to provide anindication of temperature of the device so that increased current can besupplied to the device when it has a low temperature.

It is an object of the present invention to provide apparatus for use inpower switching and a method of controlling switching of a circuitprotection device.

According to one aspect of the present invention there is providedelectrical apparatus of the above-specified kind, characterised in thatthe apparatus includes a processor that calculates the resultant heatgain and heat loss of the semiconductor device over a period of time ata plurality of different nodes within the device, that the processorcalculates the temperature of the device at a plurality of differentnodes in the device at the end of that time, and that the apparatusrestricts flow of current through the device if the calculatedtemperature of the node with the highest calculated temperature exceedsa predetermined temperature.

The apparatus preferably restricts current flow through the device byopening the device. The semiconductor switching device preferablycomprises a semiconductor die mounted on one side of a substrate, thetemperature sensor being mounted on the opposite side of the substrate

According to another aspect of the present invention there is provided amethod of protecting a semiconductor circuit protection devicecomprising the steps of measuring the temperature in the vicinity of thedevice, determining the power dissipated by the device, determining thetemperature of the semiconductor device from the power dissipated by thedevice and the measured temperature in the vicinity of the device,comparing the determined temperature with a maximum permittedtemperature, and restricting current flow through device if the maximumpermitted temperature is exceeded, characterised in that the resultantheat gain and heat loss of the semiconductor device is calculated over aperiod of time at a plurality of different nodes within the device, thatthe temperature of the device at a plurality of different nodes in thedevice is calculated at the end of that time, and that the flow ofcurrent through the device is restricted if the calculated temperatureof the node with the highest calculated temperature exceeds apredetermined temperature.

Electrical apparatus and its method of operation, in accordance with thepresent invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 illustrates the apparatus schematically; and

FIG. 2 is a side elevation view of the semiconductor device in moredetail.

The apparatus includes electrical equipment 1 and associated wiring 6connected to a power source 2 via a solid state semiconductor device inthe form of a power switch 3. The power switch 3 comprises asemiconductor die 30 secured to the top of a base substrate 31 by meansof a bond layer 32. The substrate 31 may be arranged to act as a heatsink. The die 30 includes various electronic junctions used to preventor enable current flow through the switch 3. The switch 3 isencapsulated in the usual way. A current sensing device 4 supplies asignal proportional to the current flowing through the power switch 3and this is converted to a convenient digital form by means of ananalogue-to-digital converter 9 at an appropriate regular sampling rate.A voltage sensing device 5 supplies a signal proportional to the voltageacross the power switch 3, this being converted to digital form by theanalogue-to-digital converter 9.

A temperature sensing device 10 is mounted by means of an adhesive, orthe like, to the underside of the base substrate 31 and supplies asignal proportional to the temperature of the power switch 3. Thetemperature sensor 10 cannot be mounted directly at the semiconductorjunction, which is the region most susceptible to excessive temperature,so it only provides a temperature signal representative of thetemperature in the vicinity of the relevant part of the device. Thistemperature signal is converted to digital form by means of theanalogue-to-digital converter 9.

The analogue-to-digital converter 9 could be implemented as threeseparate devices or as a single analogue-to-digital converter with amultiplexing arrangement to enable more than one signal to be converted.Each output sample of the analogue-to-digital converter 9 is used as anew data input to a processor 15, which calculates the temperature atthe junction of the semiconductor die 30 from the power dissipated inthe switch 3 and the temperature in the vicinity of the junction, asindicated by the output of the temperature sensor 10. More particularly,the processor 15 carries out the following steps in turn for each newdata value:

1. Calculate the gross power dissipation within the solid statesemiconductor switch 3 as a function consisting of the product of thedigital representation of the current flowing through the power switch 3and the digital representation of the voltage across the switch 3. Itmay be convenient to use a look-up table to perform this calculation orto scale the inputs or result using an appropriate scaling factor.

2. With the digital value representing gross power dissipation used asan input, calculations conforming to equation 1 are now performed:##EQU1## where T is the calculated temperature of the node.

q_(i) is the power delivered to node.

R_(ij) is the thermal resistance between two adjoining nodes.

C_(i) is the value of thermal capacitance for the node.

Δt is time step.

p+1 is the index indicating the predicted temperature at the end of thetime step.

p is the index indicating the temperature calculated for the previoustime step.

The node of interested is designated with the subscript "j" and theadjoining node with the subscript "i"

The semiconductor structure is decomposed into a matrix of volumeelements, where each volume element can be viewed as a node which isconnected by thermal resistance's to its adjoining neighbours.

Global assembly of equations conforming to Equation (1) are used toestimate the temperature of the semiconductor switch at discrete pointsin its structure. At a suitable point in the semiconductor nodaldecomposition the calculated temperature T_(j) ^(P) is substituted bythe converted value of temperature given by the temperature sensingdevice, thus providing a reference temperature of the ambienttemperature conditions.

3. Compare the node with the maximum temperature to the maximumpermitted temperature.

4. Command the semiconductor protection device 3 to open, haltingcurrent flow to the electrical load 1 if the node with the maximumtemperature exceeds the permitted temperature.

The above method estimates the maximum temperature of the junction ofthe semiconductor protection device structure using the finitedifference technique. Alternatively, the method of temperatureestimation could be implemented using the finite element or transmissionline matrix techniques.

The processor 15 could be a microprocessor or microcontroller.Alternatively, the processor could be implemented by dedicated orprogrammed logic devices connected in a circuit.

Although the above techniques can function satisfactorily, because theyoperate iteratively, the accuracy and continuous nature of its operationis dependant on the program execution time. Depending uponcircumstances, it may be necessary to implement an additional controlmechanism to the semiconductor protection device 3 to limit damage inthe event of very high current. Similarly, depending upon circumstances,it may be desirable to input information from other sensors orinformation via a suitable interface, which the processor will considerto modify its control to the semiconductor switching device. An exampleof this would be a temperature sensor strategically located next to theload, which could be used to modify the amount of power supplied to theload in the event that the load temperature exceeds pre-set limits. Analternative example would be information from another controller.

When an excess temperature is detected, it may not be necessary in everyapplication to open the semiconductor switch 3 in order to protect,since it is only necessary to restrict power dissipated by the device toa safe level. This could be done by reducing the current, such as byconnecting a resistive component in shunt with the device, by removingsome of the load, by using the semiconductor switch to limit thecurrent, or by other means. Alternatively, the current flow through thedevice could be completely prevented by opening some other switch, orfuse, in series with the semiconductor devices.

It may also be desirable for the processor to be provided with asuitable interface to communicate information known to it to externalsystems for more comprehensive system control. Such information, forexample, may include present current flow, voltage across thesemiconductor protection device, power dissipated within the protectiondevice, and status: on, off, tripped.

I claim:
 1. Electrical apparatus including a semiconductor switchingdevice (30), an arrangement (4, 5, 9, 15) that derives a measure ofpower dissipated by said device, a sensor (10) that provides a measureof the temperature in the vicinity of the device, and means to restrictflow of current through the device (3) in response to a combination ofthe measure of power and the measure of temperature, characterised inthat the apparatus includes a processor (15) that calculates theresultant heat gain and heat loss of the semiconductor device (30) overa period of time at a plurality of different nodes within the device(30), that the processor (15) calculates the temperature of the deviceat a plurality of different nodes in the device at the end of that time,and that the apparatus restricts flow of current through the device ifthe calculated temperature of the node with the highest calculatedtemperature exceeds a predetermined temperature.
 2. Electrical apparatusaccording to claim 1, characterised in that the apparatus restrictscurrent flow through the device by opening the device (30). 3.Electrical apparatus according to claim 1 or 2, characterised in thatthe semiconductor switching device comprises a semiconductor die (30)mounted on one side of a substrate (31), and that the temperature sensor(10) is mounted on the opposite side of the substrate.
 4. A method ofprotecting a semiconductor circuit protection device (30) comprising thesteps of measuring the temperature in the vicinity of the device,determining the power dissipated by the device (30), determining thetemperature of the semiconductor device from the power dissipated by thedevice and the measured temperature in the vicinity of the device,comparing the determined temperature with a maximum permittedtemperature, and restricting current flow through device if the maximumpermitted temperature is exceeded, characterised in that the resultantheat gain and heat loss of the semiconductor device (30) is calculatedover a period of time at a plurality of different nodes within thedevice (30), that the temperature of the device at a plurality ofdifferent nodes in the device is calculated at the end of that time, andthat the flow of current through the device is restricted if thecalculated temperature of the node with the highest calculatedtemperature exceeds a predetermined temperature.